Reflectance spectra of possible Phobos analogue materials as a support for interpretation of Mars Express data.

Abstract

Origin and mineralogical composition of Phobos and Deimos remain a mystery. Although telescopic observations provided some clues, up to now the only spatially resolved spectral data were obtained by Phobos 2 spacecraft in 1989. Serious calibration problems precluded unambiguous interpretation of those data in terms of mineralogy and composition. Mars Express mission has delivered Phobos images of unprecedentedly high spatial resolution (HRSC camera) and optical spectra in a wide spectral range from visual to thermal infrared (OMEGA and PFS spectrometers). A spectral library of relevant analogue materials is essential to interpret the spectral/colour data in terms of mineralogy. Here we present laboratory reflectance spectra of several materials which extend available spectral data for possible Phobos analogues and may help to extract compositional information from the spectra provided by the instruments onboard Mars Expess spacecraft. Optical properties of Phobos regolith may be significantly modified by micrometeoritic bombardment. We present reflectance spectra of a CM2 carbonaceous chondrite Mighei and artificial mineral mixtures irradiated with a microsecond pulsed laser to simulate optical effects of micrometeoritic bombardment. We show that such a processing causes dehydration, modifies spectral shape in the visible and near-infared, and enhances spectral signatures of olivine in the thermal infrared. In addition, we present reflectance spectra of various size fractions of the complex Kaidun microbreccia meteorite. Phobos has been suggested as a possible parent body of this unique meteorite (Zolensky and Ivanov 2003, Chem. Erde 63, 185-246).